You shouldn't have any problems with synchronized methods in helper classes, but it seems to be a very strange request. Each EJB instance will be single threaded, so there should be no need to synchronize at your helper class methods.
What are you trying to do?
What I was trying to do is make the helper a singleton to be shared by stateless session beans. Where the helper would have a cache and manage shadow objects from my database. I think this is all a no no and a kludge - probably what I should do is just use entity beans from a session bean facade.
Any ideas or comments?
There's alot of information available on singletons in J2EE, check the resources on theserverside.com for example.